Coaxial, dual unit, electrodynamic loud-speaker



Jan. 11,v H F OLSON ETAL coAxIAL, DUAL UNIT, ELECTRODYNAMIC LoUmsPEAxER Filed July 2l, 1950 gianni ESTQN BY al ATTO R N EY HARRY F., LnN

United States Patent O COAXIAL, DUAL UNIT, ELECTRODYNAMIC LOUD-SPEAKER Application July 21, 1950, Serial No. 175,112

6 Claims. (Cl. 179-1155) The present invention relates to loudspeakers, and more particularly to an improved magnetic structure for coaxial, dual unit, electrodynamic loudspeakers of the type disclosed, for example, in our copending applications Serial Numbers 51,962 and 90,464, the application secondly identified having issued as Patent No. 2,539,672 on January 30, 1951, filed, respectively, September 30, 1948 and April 29, 1949.

In sound translating apparatus of this kind, a plurality of diaphragms are employed, each of which functions to reproduce sound waves in a different frequency range. Voice coils, which are suspended in separate air gaps provided by a magnetic field structure, are attached to the diaphragms for producing vibratory movement thereof, the apparatus operating to reproduce sound in a manner well known in the art.

As disclosed in the aforesaid copending applications, it has been found advantageous to supply magnetic ux to two air gaps from a single magnetic source rather than from separate sources. Since the magnetic tlux producing element is a major item in loudspeaker production cost, especially when. it is of the permanent magnet type, it will be apparent that further advantages in lower cost production may be derived from increasing the eiciency of the magnetic structure and thereby reducing the size of the magnetic energizing unit required for a duo-cone loudspeaker having a given output.

Therefore, a primary object of the present invention is to provide an improved magnetic structure for a coaxial, dual unit, electrodynamic loudspeaker.

It is another object of the present invention to improve the efficiency of the magnetic circuit in a duo-cone, dynamic loudspeaker.

It is a further object of the present invention to provide an improved magnetic structure for a duo-cone dynamic loudspeaker which permits a greater percentage of useful flux to be distributed to the air gaps relative to that dissipated in leakage.

It is also an object of the present invention to provide an improved, simplified magnetic structure for a duocone, dynamic loudspeaker which is highly efficient in use, and which will effect a saving in cost of production,

The improved magnetic field structure is particularly useful for a dynamic loudspeaker having a plurality of separately driven, overlapping, nested cone type diaphragms. In accordance with this invention, there are provided a plurality of additional apertures in spaced array around the central aperture of one of two parallel, spaced apart outer pole piece members. Portions of the voice coil form associated with one of the diaphragms are inserted through the additional apertures to permit disposing the voice coil for that diaphragm within an air gap provided in the other outer pole piece member. With this arrangement, a closer spacing of the two outer pole piece members can be made with the result that the structure has greater magnetic efficiency.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description of a single, preferred embodiment thereof when read in connection with the accompanying drawing, in which Figure 1 is a central, sectional view taken axially through a duo-cone, dynamic loudspeaker embodying the present invention, portions of the dishpan support and one of the diaphragms being cut away,

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Figure 2 is a sectional View taken on the line 2 2 of Figure 1,

Figure 3 is a perspective view of the larger voice; coil zltnd voice coil form of the loudspeaker shown in Figure and Figure 4 is a wiring diagram of an electrical circuit corresponding to the magnetic network of the loudspeaker magnetic system shown in Figure l.

Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown a dynamic loudspeaker 1 comprising two nested, overlapping, truncated cone diaphragms 3, 5 mounted in concentric, spaced apart relation and a magnetic field structure 7 The cone diaphragms 3, 5 are of different sizes, the large diaphragm 3 comprising a vibratory, sound generating section 4 which is particularly adapted for producing sound waves within a predetermined range of low frequencies, while the small diaphragm 5 comprises a vibratory, sound generating section 6 which is particularly adapted for producing sound waves within a predetermined range of high frequencies. The diaphragms 3, 5 `also include voice coil sections consisting of cylindrical or tubular voice coil forms 9, 11 of relatively greater and lesser diameters and attached, respectively, to the apex ends of the diaphragms 3, 5. Voice coils or windings 13, 15 are disposed respectively on and adjacent the voice coil forms 9 and 11 remote from the sound generating sections of the diaphragms. Any suitable support may be provided for mounting the diaphragms 3, 5 so that they can move substantially only in axial directions and be free to vibrate in well known manner to produce.` sound waves.

The magnetic field structure 7 comprises an outer pole piece unit 17, a central pole piece 19,. and a magnetic core 21 connecting the pole piece unit 17 and the central pole piece 19. The outer pole piece unit 1'7 is constituted of a magnetically permeable material and comprises a yoke 23 and a pair of outer pole piece members 25, 27 having, respectively, central apertures 29, 31 of different diameters. The outer pole piece member 25, sometimes referred to herein as the first pole piece, has the larger diameter central aperture 29 and comprises an annular plate mounted to span the ends 33 of the yoke 23, the pole piece 25 being fastened to the yoke 23 in any suitable manner. The other outer pole piece member 27, sometimes referred to herein as "me second pole piece, comprises an inverted, cup-like member. "lne cup-like second pole piece 27 is mounted on the rst pole piece 25 with the peripheral flange portion 34 thereof in contact with the upper side or face of the first pole piece 25, as shown in Figure l, its annular, plate-like portion 35 surrounding the central aperture 31 and being spaced from the iirst pole piece 25. The central apertures of both pole pieces 25, 27 are aligned along a common axis which corresponds to the common axis of the diaphragms The magnetic core 21 comprises a cylindrical member of suitable permanent magnet material. The core 21 is mounted centrally within the yoke 23 with one end 36 thereof attached to the yoke base 37. The central pole piece 19 comprises a member of magnetically permeable material having one end 39 thereof mounted on the opposite end 41 of the core 21. The central pole piece 19 extends through both central apertures 29, 31 in spaced relation to the pole pieces 25, 27 so as to provide separate annular air gaps 43, 45 therebetween. Since the central apertures 29, 31 are, respectively, of greater and lesser diameters, the portion of the central pole piece disposed within the larger air gap 43 is larger in diameter than its opposite end portion disposed within the smaller air gap 45. Thus, the central pole piece 19 may be said to comprise a member of inverted T formation in cross section, as shown in Figure 1.

The voice coil 13 mounted on the larger voice coil form 9 and the voice coil 15 mounted on the smaller voice coil form 11, are suspended, respectively, within the larger and smaller air gaps 43, 45. Flexible members 47, 49 are provided for supporting, respectively, the voice coil forms 9, 11 and for maintaining them centered Within the air gaps while permitting them to move axially. Since the larger voice coil 13 is located on the opposite side of the smaller pole piece member 27 from that on which. the.

larger diaphragm 3 is disposed, a plurality of kadditional apertures 51 are provided in the pole piece 27 in spaced array around the central aperture` 31. Web sectionsfremain between the apertures 51, as shown. A s shown particularly in Figure 3, the end portion of thegreater diametervoice coil form 9 to which the diaphragm 3is attached is crenelated or cut away to form, at one end thereof, a plurality of circumferentially spaced, longitudinally extended portions or elements 53 of arcuate cross section. In assembly, the transversely arcuate extensions 53 are inserted through the apertures 51so that the voice coil 13 is disposed within `the larger air gap43 located on one side of the outer pole piece member 2,7 and the free ends of the extensions 53 are on the opposite side of the outer pole piece member.

The magnetic circuit of the loudspeaker 1 may be represented by a circuitdiagram as shownin Figure 4, in which:

Mr--magnetomotive force of the magnet, in gilberts;

Mz=magnetomotive force drop through the yoke and the outer portion of the larger outer pole piece up to the region where the smaller outer pole piece is supported on the larger outer pole piece, in gilberts;

Mszmagnetomotive force` across the smaller air gap, in gilberts;

M4=magnetomotive force` drop through the central pole piece and the smaller outer pole piece, in gilberts;

M=magnetomotive force across the larger air gap, 1n gilberts;

R1=reluctance drop in the yoke and the outer portion of the larger outer pole pieceup tothe region where the smaller outer pole piece is supported onthelarger outer pole piece;

Rzzreluctance of leakage ux pathacross the, space within the magneticeld structure` and around the magnet exclusive of the larger air gap;

Rs=reluctance of the smaller air gap;

Rareluctance of the central pole piece and thesmaller outer pole piece;

R5=reluctance ofl the larger air gap;

1=total vlines of llux delivered by the magnet, 1n maxwells; a

2=pleakage lines of ilux across thespacewithm the. magnetic eld structure and aroundthe magnet excluslv, of the larger air gap, in maxwells; a

4=total lines Aof ilux through thel smallerair gap,Y 1n` maxwells;

5=total lines of ux through thelarger air.;gap 1n rrraxwells..y

The total flux throughthe Atwo air` gapsis represented by theV equation s,

The reluctance ofthe combination of the, smallerair. gap.

in series with the central pole piece and this combination.

in shunt with,` the. larger air gap is.;represent edby xthe equation:

Rs -l- Ri -l- Ru Theequationrepresenting the magnetomotive forceof the larger air gap is:

where A3=area of the smaller air gap, in centimeters.

From the above equations,` it will be recognized by persons skilled inthe artthat any practical ratio of` flux densities may be obtained by arsuitable selection ofthe values for R4. Withthe arrangement of the outer pole piece membersV 2,5, 27 asv described above, the value of R4 can be varied by altering the diameter of the portion of the central pole piece 19 below the small air gap. The magnetic eiliciency of the structure is high because less liux is dissipated inrleakage. Therefore, a greater percentage of useful ux will be supplied to each of the air gaps. The reason less ilux is dissipated in leakage, as compared to other multiple gap structures, is that the large and small pole pieces and outer structures operate at-.substantially the same magnetic potential. With the increase in magnetic elciency, itis not necessary to employf as large a magnet to produce equiyalentiluxv densities v`in the two air gaps. A- considerable saving in cost ofproduction will result because a` major` portion of the material,l cost resides lin the magnet.

Although there is illustrated and described ,buta single embodiment ofthe present invention, it will be recognized by persons skilled in the art that various modificationsl and changesare possible within the spiritof the invention. For example, it may be found desirable -to make the large diaphragmand the large voice coil form an integral struc@ ture, in which case, portions of the voice coil form may be cutaway intermediate the `ends thereof. It may.` also be found desirable to provideslots in the small outer pole piece 27 which extend radially outwardly-from the periphery of the large central aperture insteadof arcuate open-A ings. Other changes of like nature are possiblewithinl the spirit of our present. invention. Therefore, itis desired that the particular form of the` present inventionv der.v scribed herein` shall be considered `as illustrativeandnot as limiting.

What is claimed is:

l. Awloudspeaker comprising an innertdiaphrllgfvl.VV and an outer diaphragm,y each' diaphragm includingavoice coil supporting section, means to mount` said diaphragms for movement in4 directions along,a-commonaxis,and a magnetic field structure including a entralpole piece-and a pair of outer'polerpieces, said outerV pole-pieces VeachI having a central aperturetherein, the .portions of said outer pole pieces deninggsaid central apertures being dise posed in spaced relation to each other -withgtheir4 central apertures aligned along saidcommonaxis,-said,central pole piece extending through each of said,centralfaperturesirr spaced .relation tolsaid outer pole piece portionsto-,pro air gaps.therebetween,` one of saidppole pieces having-5a plurality of additional apertures invspackedarray.aroundjts said central, aperture, portionsmf A'said,.,vc, ice.coil supporting 'sections beingdisposed in sepa,r,ateor1e s .ofisaidV air;

f gaps,` androther portionsof said voice lcoil supportingsegf.

tion of saidV outer, diaphragmextending ,thrmlgll.fsaidgaddi;` tionalaper-tures in saidrOIle-polefpiece-.

2., yAD loudspeaker comprising :an inner',diaphragm;and.` an outer, diaphragm, each'diaphragm in cludingiarsollndI generating section anda voice coilection ,connectedto said` sound generatingsectionnmeansfftofmount said'rrdia; phragms with `their `longitudinal axes in :commen-f-,arrdral magnetic fieldy structure `comprising ra core,A `a pain ffipolf:` piecesV .mounted` in axially lspaced relationzto reach, other; saidpole pieces -each lhavingfcentral -aperturesrandybeingcore. to provide separateaxially spaced@ innerfgandv routerA alr gaps., @neef Said pole pieces having .a pluralitrfoiop ngstherein in lspacedfarray,-aroundgits;,centralY apertur ,l

pole piece.

3. A coaxial, dual unitloudspeaker comprising an inner,

diaphragm and an outer diaphragm, each diaphragrrrin cluding a voice coil supporting section, means forrnountf lng sa1d voicencorl. supporting sectionsforgrnovernentiirr directionsalong.acommon axis,l and a magnetic-fieldA struts-.-` ture including a centralpcore, alirsttourenfpoleipieceshave:

ing a plate portion provided with a relatively large central aperture, a second outer pole piece having a plate portion spaced from said first mentioned plate portion and provided with a relativelyI small central aperture, said apertures being axially aligned, and said central core being disposed within each of said central apertures in spaced relation to said rst and second outer pole pieces to provide, respectively, iirst and second air gaps therebetween, said second outer pole piece having a plurality of additional apertures therein in spaced array about its central aperture, at least a portion of said coil supporting section of said inner diaphragm being disposed within the air gap provided by said second outer pole piece, the voice coil supporting section of said outer diaphragm having at least a portion thereof disposed in the other of said air gaps and at least another portion thereof extending through said additional apertures.

4. A loudspeaker having a pair of nested inner and outer diaphragms, each of said diaphragms having a tubular extension, a voice coil on each extension, a magnetic eld structure having a central pole piece, said tubular extensions being disposed over said central pole piece, a second pole piece of opposite magnetic polarity with respect to said central pole piece surrounding said central pole piece and the tubular extension of said inner diaphragm, a third pole piece of opposite magnetic polarity with respect to said central pole piece surrounding said central pole piece and the tubular extension of said outer diaphragm, said second pole piece having a plurality of circumferentially spaced arcuate slots having a common center of curvature, web sections remaining in said second pole piece between said slots, said tubular extension of said outer diaphragm having axially disposed apertures therein, and said apertures accommodating said web sections between said spaced slots.

5. A loudspeaker having a diaphragm, said diaphragm having a tubular extension, a voice coil on said extension, a magnetic eld structure having a central pole piece, said tubular extension being disposed over said pole piece, a second pole piece associated with said magnetic structure, said magnetic structure having a third pole piece of opposite magnetic polarity with respect to said central pole piece surrounding said central pole piece and the tubular extension of said diaphragm, said second pole piece having a plurality of circumferentially spaced arcuate slots having a common center of curvature, web sections remaining in said second pole piece between said slots, said tubular extension of said diaphragm having axially disposed apertures therein, and said apertures accommodating said web sections between said spaced slots.

6. A diaphragm having a body portion and a tubular extension for use in a loudspeaker, said tubular extension extending along the axis of said body portion, said tubular extension having a plurality of circumferentially arranged longitudinally disposed open slots, and said tubular extension being joined to said body portion substantially at the ends of said open slots.

References Cited in the iile of this patent UNITED STATES PATENTS 1,848,433 Pridham Mar. 8, 1932 1,869,230 Timmons July 26, 1932 1,891,404 Engholm Dec. 20, 1932 1,923,924 Engholm Aug. 22, 1933 1,951,692 Evans Mar. 20, 1934 2,037,811 McMurtrey Apr. 2l, 1936 2,539,672 Olson et al Jan. 30, 1951 

